The invention concerns a force augmenting device, such as a clutch-servo for engaging and disengaging vehicle clutches, which device constitutes, a working or slave device arranged for fluid connection with a master cylinder to be operated by the vehicle's driver, comprising a working cylinder unit which has a piston installed in a working cylinder, housing or the like, which piston is connected to the unit which is to be actuated by the force or pressure augmenting device, and a control valve unit for the working cylinder, with a valve body which is installed in a valve housing with a supply channel and a return channel for a servo pressure fluid where the master cylinder has a fluid connection with the working cylinder unit and the control valve unit in such a way that the fluid from the master cylinder simultaneously exerts a force on the piston and on the valve body, in order to try to move the piston and the valve body respectively when the driver operates the master cylinder, and the valve body is arranged so as to provide communication between the working cylinder's cylinder space and the return channel for the servo pressure fluid when the master cylinder is not in operation, there being no functionally determined mechanical connection between the working cylinder unit and the valve unit.
A device of the above-mentioned type is described in U.S. Pat. No. 3,182,443, but since it comprises very many components it will thus be liable to failure during use. Furthermore these components have to be produced with fine tolerances, which makes this an expensive arrangement. The valve device is installed inside the piston, which therefore also has to be dismantled if the valve unit fails.
A further device of the above-mentioned type is also known from DE 24 40 039, where the piston is mechanically connected to a valve body which follows the movement of the piston when the clutch pedal is moved. Due to the substantial movement of the valve body, this will be subject to relatively heavy wear and tear, and as the working cylinder and the valve unit constitute a mechanical unit, both have to be dismantled if only one of the devices should fail. Moreover, the valve sealing rings are subject to great differential pressure, which makes the valve body hard to move.
Moreover, from EP patent application No. 0 310 733 it is known that a master cylinder may be hydraulically connected to a working cylinder and a control valve, the pressure from the master cylinder controlling the control valve which in its turn regulates the servo pressure fluid's access to a second working cylinder. The control valve which is shown, however, is a seat valve. Valves of this kind are unsuitable for proportional control purposes, and it has been shown that seat valves used in this connection cause relatively severe hysteresis while in addition the force required to operate the master cylinder will be highly dependent on the pressure of the servo pressure fluid.
The object of the invention is to provide a clutch-servo of the type described which is not encumbered with the above-mentioned disadvantages.